Process for preparing ammonia

ABSTRACT

In a process for preparing ammonia from hydrogen and nitrogen the synthesis gas mixture is produced by partial oxidation, in the presence of a suitable catalyst, at a pressure of from 35 to 150 bar and temperatures of from 850°-1200° C. at the exit of the partial oxidation zone, followed by removal of the carbon oxides and water from the gaseous effluent of the partial oxidation zone. The air used for the catalytic partial oxidation is supplied in such a quantity that the molar ratio of hydrogen to nitrogen in the synthesis gas is between 2.5 and 3 to 1 and is enriched with such a quantity of oxygen that the total quantity of oxygen is sufficient to effect the required degree of hydrocarbon conversion.

This invention relates to a process for preparing ammonia.

In current processes for the preparation of ammonia the synthesis gas isusually prepared by steam-reforming or partial oxidation of ahydrocarbon feed stock, which can be a liquid or a gaseous hydrocarbonor mixture of hydrocarbons, e.g. naphta or natural gas. A process of thetype wherein the synthesis gas is obtained by partial oxidation is knownfrom P. H. Brook: Ammonia Plant Revamping, Proceedings of the FertilizerInternational Conference 1983, p. 159-175. In this process natural gasis partially oxidized with air in a Texaco gasifier and the quantity ofnitrogen introduced in the process with the air, in excess of thequantity stoichiometrically required for the conversion of the hydrogenformed to ammonia, which excess may be about 200%, is removed in acryogenic separation section. The feed gas and the process air arepreheated to about 590° C. and about 815° C., respectively, in aseparate furnace by combusting a suitable fuel, such as natural gasexpanded to atmospheric pressure. The quantity of oxygen must besufficient to attain the required degree of hydrocarbon conversion andthe feeding thereof as air at the reaction pressure to the partialoxidation zone involves the compression and the heating of the excessintrogen and other components of the air, which heating occurs bycombustion of natural gas in the preheating and the partial oxidationzones. However, the compression energy and the heat required thereforcan be recovered only partially. Moreover, the non catalytic partialoxidation process used necessitates a carbon removal step to remove thesolid carbon formed in the partial oxidation zone and entrained in thegaseous effluent therefrom. The temperatures in the partial oxidationreactor are high so that at the exit the temperature is about 1260° C.

The principal object of the present invention is to provide a processfor preparing ammonia the energy consumption of which is lower than theenergy consumption of the known process referred to. More particularlythe present invention is aimed at a process for preparing ammonia inwhich the presence of a large excess of nitrogen in the ammoniasynthesis gas is obviated so that a cryogenic removal of excess nitrogencan be dispensed with. A still further particular object is to provide aprocess in which the heat generated in the partial oxidation zone isused in an efficient manner.

These, and other objects which will become clear from the detaileddescription below, are attained in a process for the preparation ofammonia from hydrogen and nitrogen comprising the steps of:

(a) feeding to a first reaction zone, at a suitable pressure, a streamof air and a stream of hydrocarbon or a mixture of hydrocarbons to formby partial oxidation a gas mixture comprising hydrogen, nitrogen, carbonoxides, water and unconverted hydrocarbon material;

(b) shift conversion of carbon monoxide contained in the gas mixtureobtained in step (a) into carbon dioxide and hydrogen and removingcarbon dioxide and water from the gas mixture resulting from said shiftconversion,

(c) feeding the synthesis gas mixture resulting from step (b) to asecond reaction zone to partially convert hydrogen and nitrogencontained in said synthesis gas mixture to ammonia;

(d) separating ammonia from the gaseous effluent from said secondreaction zone;

(e) recycling at least a portion of the gas mixture remaining after theseparation of ammonia in step (d), in which process, according to theinvention,

the partial oxidation of step (a) is carried out in the presence of asuitable catalyst at a pressure of from 35 to 150 bar, and temperaturesof from 850°-1200° C. at the exit of the first reaction zone, thequantity of air fed to the first reaction zone is such that the molarratio of hydrogen to nitrogen in the gas mixture resulting from step (b)is between 2.5 and 3 to 1 and an additional quantity of oxygen is fed tothe first reaction which oxygen together with the oxygen contained inthe said quantity of air is sufficient to effect the required degree ofhydrocarbon conversion.

The catalytic partial oxidation of hydrocarbons is known per se. Itcomprises passing the hydrocarbon feed material over a suitablecatalyst, at a pressure between about 35 and about 150 bar andtemperatures increasing toward the exit of the reaction zone betweenabout 850° and 1200° C. In partial oxidation processes the hydrocarbonsare first oxidized with a limited quantity of oxygen whereby carbonmonoxide and carbon dioxide are formed and heat is released. Incatalytic partial oxidation processes the heat released in thisoxidation is used to catalytically convert hydrocarbons which have notyet been oxidized, in the presence of steam. Therefore in the latterprocesses no heat need be supplied from an external source through thewall of the reaction zone. Up till now non-catalytic partial oxidationhas been used virtually exclusively for the conversion into synthesisgas of higher hydrocarbons such as fuel oil or cracked petrol and steamreforming has been considered more suitable for processing lowerhydrocarbons. However, non-catalytic partial oxidation must be carriedout at considerable higher temperatures between 1250° and 1400° C. andeven higher. The applicant has now found that the application ofcatalytic partial oxidation in an ammonia synthesis starting from lowerhydrocarbons e.g. those containing 1-3 carbon atoms, when carried outwith air enriched with a sufficient quantity of oxygen does offerspecial advantages with respect to steam reforming and non-catalyticpartial oxidation:

catalytic partial oxidation can be carried out at higher pressures thansteam reforming and as a result less compression energy is required tocompress the synthesis gas mixture to the pressure required in theammonia synthesis. This is particularly advantageous if the startingmaterial is natural gas already available at high pressure, because inthat case no pressure reduction is necessary;

catalytic partial oxidation can be carried out at lower exittemperatures than non-catalytic partial oxidation with the result thatless oxygen and feed gas are required for the conversion of thehydrocarbons;

As only so much air is supplied to the partial oxidation zone as isnecessary to obtain a synthesis gas mixture containing hydrogen andnitrogen in the required ratio the amount of energy for compression ofthe air and the synthesis gas is substantially reduced. The totalquantity of gas to be handled in the process is reduced accordingly.

carbon removal, which is necessary with non-catalytic partial oxidation,can be dispensed with;

a smaller amount of water is required than in steam reforming processesto attain the required conversion in the partial oxidation zone as theconversion takes place partly according to the reaction

    CH.sub.4 +1/2O.sub.2 +N.sub.2 →CO+2H.sub.2 +N.sub.2.

This reaction is exotherm and releases 37 MJ/kmole CH₄. Hence, lesssteam need be supplied and less energy is required to decompose thissteam. The molar steam-to-carbon ratio thus can be varied between 1.0and 3.0. Advantageously, this ratio is selected between 1.5 and 2.5 inorder to have the full profit the catalytic partial oxidation offers.

The principles of the process according to the invention will bedescribed in more detailed with reference to the accompanying drawingwhich shows a schematic flow diagram of the various process steps.

Via line 1 air is supplied to an air separation unit 2 wherein a gasmixture mainly consisting of e.g. about 40% by volume oxygen and for therest nitrogen is produced. The remainder of the air is discharged via 3and processed for further use, if required. The proportion of oxygen inthe gas mixture to be used is selected dependent fo the conditions atwhich the partial oxidation is effected. The mixture of oxygen andnitrogen obtained in air separation unit 2 is conveyed, via line 4, tocompressor 6 together with a stream of air flowing through line 5. Thequantity of air is so chosen as to result in a synthesis gas mixturehaving the required ratio hydrogen to nitrogen, which ratio is between2.5 and 3 depending on the ammonia synthesis process applied. Aftercompression to a pressure somewhat hydrocarbon higher than the selectedpressure at which the partial oxidation is carried out, which pressureis between 35 and 120 bar, advantageously between 45 and 80 bar, the airenriched with oxygen is passed to a saturator 27 where it is saturatedwith water in a manner to be discussed later. The saturated enrichedair, to which, via line 7, a further quantity of water or steam may beadded, as required, is heated in one or more heaters 8 to a temperatureof from 450°-900 ° C. and introduced in a first reaction zone i.e. thepartial oxidation reactor 10 via line 9.

The hydrocarbon feed material is supplied via line 11. The hydrocarbonfeed material which may contain 1-3 carbon atoms per mole isadvantageously natural gas, but other gaseous hydrocarbons and evennaphta may be used. Water is added in saturator 12. If necessary, thegaseous hydrocarbon material is compressed to somewhat above theselected reactor pressure. On the other hand, if, as is the case at anumber of locations, it is available at high pressure in generalreduction of the pressure is not necessary as the catalytic oxidationcan be carried out at high pressures of from 35 to 150 bar.Desulphurization of hydrocarbon feed material is only necessary if thecarbon dioxide produced in a subsequent carbon dioxide removal sectionshould contain substantially no sulphur compounds and may take placeprior to introduction in saturator 12 or in any other suitable part ofthe process. A further quantity of water or steam may be added as neededvia line 13. The hydrocarbon feed steam which has substantially beensaturated with water is subsequently preheated in heater 14 to atemperature which advantageously is in the range of from 450° to 750° C.The preheating of both the enriched air stream and the hydrocarbon feedstream to the indicated high temperature level results in a substantialreduction of the quantity of heat to be produced in the partialoxidation reactor 10 and consequently a smaller quanity of additonaloxygen need be supplied by the air separation unit 2 and compressed incompressor 6. The preheated hydrocarbon feed stream enters, via 15, thepartial oxidation reactor 10 in which a first part of it is combusted,with the aid of the enriched air, to carbon monoxide, carbon dioxide andwater and this mixture is next passed through a catalyst bed 16comprising a suitable catalyst such as, e.g. a nickel-containingcatalyst, in which most of the hydrocarbons still present are convertedto carbon monoxide, carbon dioxide and hydrogen.

Thus an effluent gas mixture is obtained comprising hydrogen, nitrogen,carbon oxides, water vapour, inert gases, such as argon and helium, andunconverted hydrocarbons. The temperature of this effluent gas mixtureis between 850° and 1200° C. depending on the process conditions,starting material used etc. and at a reactor pressure of about 55 barwill between 900° and 1050° C.

In the embodiment shown the effluent gas mixture is first passed througha waste heat boiler 18 via line 17 to generate steam from water suppliedthrough line 42, the steam being carried off via 19, and then throughpreheater 14 to preheat the hydrocarbon feed material to a temperatureof e.g. about 650° C. Optionally the waste heat boiler 18 may be omittedand a larger proportion of the heat content of the effluent gas mixturemay be used to preheat the hydrocarbon feed stream to a highertemperature of say 700° to 750° C.

The partially cooled effluent gas mixture leaving preheater 14 is passedto a shift conversion section 21 which normally comprises a hightemperature stage and an low temperature stage, well known in the art,wherein the carbon monoxide is catalytically converted with steam tocarbon dioxide. If in the partial oxidation step a comparatively lowsteam to carbon ratio is maintained the steam content in the gas mixturewill be accordingly low, but it will be understood that by properselection of the catalysts for both stages a satisfactory degree ofconversion can still be obtained.

The raw synthesis gas mixture leaving shift conversion section 21 isfurther cooled in cooling unit 22 to condense most of the water itcontains. This condensation takes place at a temperature level enablingthe heating of a stream of process condensate and/or water supplied vialines 20 and 46 to the heat exchange elements 47 and the evaporationthereof in the saturators 12 and 27 to which it is supplied via pump 23and lines 24 and 25, respectively. In this way the heat released bycooling and condensation is used in an efficient manner by takingadvantage of the relatively low partial water vapour pressures in thehydrocarbon feed stream and the enriched air stream which require onlylow evaporation temperatures. The water which is not evaporated in thesaturators 12 and 27 is recycled via lines 43 and 44 to cooling unit 22for reheating.

The raw synthesis gas mixture depleted of most of the water is furthertreated in a known way in section 45 to remove substantially all thewater remained in it, the process condensate formed thereby is recycledto cooling unit 22 via line 46 for reuse in the saturation of thehydrocarbon feed stream and the enriched air and the remaining gasmixture is passed to a carbon dioxide removal section and a methanationsection commonly represented by 33 in which carbon dioxide is removedfrom the synthesis gas mixture, e.g. by selective absorption, andsubsequently the carbon monoxide and carbon dioxide are converted tomethane. The synthesis gas leaves section 33 at a pressure suitable forthe ammonia synthesis which pressure may vary between 60 and 300 bardepending on the ammonia synthesis process selected. It will beunderstood that by proper selection of the pressure at which thesynthesis gas is prepared and the pressure at which the ammoniasynthesis is operated compression of the synthesis gas can be minimizedor entirely dispensed with, which results in a substantial saving inenergy consumption.

Via line 28 the synthesis gas mixture is introduced in a second reactionzone i.e. ammonia synthesis reactor 29, together with a stream ofunconverted gases which is recycled via line 37. The ammonia synthesiseffluent stream is passed to ammonia separation section 30 in which theammonia is recovered by refrigeration. The liquid ammonia thus obtainedis passed to a pressure reduction device 48 via lines 31 and 40. In thispressure reduction device, which in the embodiment shown is an expansionturbine the pressure is reduced to a pressure somewhat higher than thatat which the partial oxidation reactor is operatued, for a reason whichwill be explained below. The expanded ammonia synthesis effluent streamflows into a flas tank 49 in which the unconverted gases containing someammonia are flashed off. The unconverted gases leaving ammoniaseparation section 30 which contain the gaseous hydrocarbons slippedthrough the partial oxidation reactor without having been converted andthe methane formed in the methanation step are passed to an absorptioncolumn 34 or a similar device via line 32 wherein they are contactedwith liquid ammonia which is taken from flash tank 40 via line 39 andrepressurized to the ammonia synthesis pressure by pump 51. In thistreatment use is made of the fact that methane and argon are more easilydissolved in liquid ammonia than hydrogen and nitrogen, and as a resulta substantial portion of the methane and argon is removed from therecycled gas mixture. This way of recovering non-converted gaseoushydrocarbons allows a more flexible operation of the partial oxidationreactor since the quantity of gaseous hydrocarbon slipping throughnon-converted is no longer critical. A large portion of the recycled gasmixture is reintroduced in the ammonia synthesis reactor 29 via line 37,another portion, advantageously 2 to 10% by volume, is recycled to thepartial oxidation reactor 10 via lines 38 and 50 and a small remainingportion may be sent to a gas scrubber 52 via line 53 for recovery of theammonia by absorption in water supplied via 54 and subsequently be usedas boiler fuel etc. or directly be purged from the process via 55. Theaqueous ammonia solution discharged from scrubber 52 may be combinedwitht the process condensate stream 46 discharged from section 45 viapump 56 and line 57. It should be noted that the purge gas stream may besplit off from another gas stream, such as e.g. the gas stream comingfrom ammonia separation section 30. Also, it is not necessary to treatthe entire gas mixture in absorption column 34, a portion may berecycled directly to the ammonia synthesis reactor 30 depending on theinert content of the feed to this reactor.

The liquid ammonia containing the unconverted gaseous hydrocarbons andinert gases absorbed in absorption column 34 is combined, via line 41,with the liquid ammonia stream leaving ammonia separation section 30 andthe combined stream is passed via line 40, to expansion rubine 48. Thegases mainly consisting of unconverted gaseous hydrocarbons separatedoff in flash tank 49 are recycled to the partial oxidation reactor 10via lines 35 and 50. A portion thereof may be purged via purge gasscrubber 52, if required. The liquid ammonia obtained in flash tank 49is partly pumped to absorption column 34 which is operated at aboutammonia synthesis pressure, by pump 51 which may be driven by expansionturbine 48, the remaining portion being passed to storage tank 60 afterthe pressure has been suitably reduced in a pressure reduction device58, such as an expansion turbine, and the gases released thereby, whichstill contain some gaseous hydrocarbons, have been separated off inflash tank 59. These gases are passed to purge gas scrubber 52 via line36.

In the embodiment shown the pressure of the liquid ammonia dischargedfrom ammonia separation section 30 is reduced in two steps so that asubstantial proportion of the recovered gaseous hydrocarbons can berecycled to partial oxidation reactor 10 without compression in aseparate compression device. It is also possible, however, to reduce thepressure in a single step to the pressure at which the ammonia is storedand to recompress the released gases to the required pressure or to usethem as burner fuel etc.

EXAMPLE

In a plant for the production of ammonia with the process according tothe invention the starting material was natural gas which was preheatedto 650° C. and air enriched with oxygen was preheated to 800° C. In thecatalytic partial oxidation reactor a molar steam-to-carbon ratio of 2and a pressure of 55 bar were maintained. The temperature at the exit ofthe reactor was 1050° C. Besides the steam generated in the saturatorsand the waste heat boiler an additional quantity of steam required onthe process was raised in a separate boiler using natural gas andprocess purge gas as fuel. The ammonia synthesis pressure was 200 bar.

The composition of the principal process streams for the production of 1ton ammonia is given in the table below. The numbers of the streamscorrespond with the reference numbers in the drawing. The quantities arein kg.

    __________________________________________________________________________        natural gas                                                                          natural gas                                                                         enriched                                                                           recycle gas                                                                         POR* Purge to                                     name                                                                              process feed                                                                         boiler fuel                                                                         air feed                                                                           to POR*                                                                             effluent                                                                           boiler                                                                             Product                                 __________________________________________________________________________    no. of                                                                            11           4 + 5                                                                              50     17  55    60                                     stream                                                                        O.sub.2          608                                                          N.sub.2                                                                           25     2     806  28    860  8                                            H.sub.2                6    144  2                                            CO                          641                                               CO.sub.2                                                                          22     2                471                                               CH.sub.4                                                                          448    35         13     8   5                                            C.sub.2 H.sub.6                                                                   72     6                                                                  Ar                20  16     36  4                                            NH.sub.3               1              1000                                    H.sub.2 O                   1071                                              Total                                                                             576    45    1434 64    3230 19   1000                                    __________________________________________________________________________     *partial oxidation reactor                                               

Based on the lower heating value the total quantity of natural gas of576+45=612 kg/ton ammonia corresponds with 28 GJ of energy per tonammonia.

COMPARATIVE EXAMPLE

In the process described in the article by Brook referred to above thenitrogen is about 200% in excess to the quantity that isstoichiometrically required in the ammonia synthesis.

    ______________________________________                                        In a process in which the partial oxidation                                                           1.1 GJ/ton NH.sub.3                                   is carried out at 70 bar                                                      the energy consumption for compression of                                     this excess nitrogen is                                                       In the cryogenic nitrogen removal                                                                     0.4 GJ/ton NH.sub.3                                   section the nitrogen is obtained                                              at 10 bar. In an expansion turbine                                            this nitrogen can produce                                                     Hence the mechanical energy to be supplied is                                                         0.7 GJ/ton NH.sub.3                                   In the process according to the invention com-                                                        0.35 GJ/ton NH.sub.3                                  prising catalytic partial oxidation with air                                  enriched with oxygen the production of the                                    additional oxygen requires in mechanical energy                               ______________________________________                                    

This results in an advantage in mechanical energy of the processaccording to the invention over the known process of 0.35 GJ/ton NH₃.This corresponds with an advantage in natural gas consumption of about 1GJ/ton NH₃.

I claim:
 1. Process for the preparation of ammonia from hydrogen andnitrogen comprising the steps of:(a) feeding to a first reaction zone,at a suitable pressure, a stream of air and a stream of hydrocarbon or amixture of hydrocarbons to form by partial oxidation a gas mixturecomprising hydrogen, nitrogen, carbon oxides, water and unconvertedhydrocarbon material; (b) shift conversion of carbon monoxide containedin the gas mixture obtained in step (a) into carbon dioxide and hydrogenand removing carbon dioxide and water from the gas mixture resultingfrom said shift conversion, (c) feeding the synthesis gas mixtureresulting from step (b) to a second reaction zone to partially converthydrogen and nitrogen contained in said synthesis gas mixture toammonia; (d) separating ammonia from the gaseous effluent from saidsecond reaction zone; (e) recycling at least a portion of the gasmixture remaining after the separation of ammonia in step(d);characterized in that the partial oxidation of step (a) is carriedout in the presence of a suitable catalyst at a pressure of from 35 to150 bar, and temperatures of from 850°-1200° C. at the exit of the firstreaction zone, an additional quantity of oxygen is fed to the firstreaction zone, controlling the quantities of air and additional oxygenwith respect to the quantity of hydrocarbon material fed to the firstreaction zone in such a manner that the molar ratio of hydrogen tonitrogen in the gas mixture resulting from step (b) is between 2.5 and 3to
 1. 2. Process according to claim 1, characterized in that in thefirst reaction zone a molar ratio steam to carbon of from 1.0 to 3 ismaintained.
 3. Process according to claim 1, characterized in that inthe first reaction zone a molar ratio steam to carbon of from 1.5 to 2.5is maintained.
 4. Process according to claim 1, characterized in thatthe catalytic partial oxidation in the first reaction zone is carriedout at a pressure of from 45 to 80 bar and temperatures of from 900° to1100° C. at the exit of the first reaction zone.
 5. Process according toclaim 1, characterized in that the gas mixture obtained in step (b) iscooled to condense at least part of the water vapour contained therein,the heat released thereby is used to evaporate water and the watervapour thus obtained is fed to the first reaction zone.
 6. Processaccording to claim 5, characterized in that the water to be fed to thefirst reaction zone is evaporated in at least one of the feed streams tothe first reaction zone.
 7. Process according to claim 1 characterizedin that in step (e) the portion of the gas mixture to be recycled is atleast partially treated to remove hydrocarbon material and inert gasestherefrom and the thus removed hydrocarbon material is at leastpartially recycled to the first reaction zone.
 8. Process according toclaim 1, characterized in that hydrocarbon material and inert gases areremoved from at least part of the portion of the gas mixture to berecycled to the second reaction zone by absorption in liquid ammonia andare separated from the resulting absorbate.
 9. Process according toclaim 8, characterized in that part of the hydrocarbon material andinert gases are removed from the absorbate by reducing the pressurethereof to substantially the pressure maintained in the first reactionzone and at least part of the thus removed gaseous mixture is recycledto said first reaction zone.